Fix(optimizer)!: ConcatWs args set incorrectly in simplify_concat

Author: georgesittas

sqlglot/optimizer/simplify.py

@@ -1296,15 +1296,15 @@ def simplify_concat(self, expression):
             sep_expr, *expressions = expression.expressions
             sep = sep_expr.name
             concat_type = exp.ConcatWs
-            args = {}
         else:
             expressions = expression.expressions
             sep = ""
             concat_type = exp.Concat
-            args = {
-                "safe": expression.args.get("safe"),
-                "coalesce": expression.args.get("coalesce"),
-            }
+
+        args = {
+            "safe": expression.args.get("safe"),
+            "coalesce": expression.args.get("coalesce"),
+        }
 
         new_args = []
         for is_string_group, group in itertools.groupby(

tests/test_optimizer.py

@@ -894,6 +894,15 @@ def test_simplify(self):
         self.assertEqual("CONCAT('a', x, 'bc')", simplified_concat.sql(dialect="presto"))
         self.assertEqual("CONCAT('a', x, 'bc')", simplified_safe_concat.sql())
 
+        # Both Databricks' and DuckDB's CONCAT_WS skip NULL args, so no CASE wrapping is needed
+        concat_ws = parse_one("CONCAT_WS(' ', a, NULL, 'b', 'c')", read="databricks")
+        simplified_concat_ws = optimizer.simplify.simplify(concat_ws)
+
+        self.assertEqual(simplified_concat_ws.args["coalesce"], True)
+        self.assertEqual(
+            "CONCAT_WS(' ', a, NULL, 'b c')", simplified_concat_ws.sql(dialect="duckdb")
+        )
+
         anon_unquoted_str = parse_one("anonymous(x, y)")
         self.assertEqual(optimizer.simplify.gen(anon_unquoted_str), "ANONYMOUS(x,y)")